1. Technical Field
The present invention relates to a fixing device and an image forming apparatus in which deterioration of image quality is restrained in an electrophotographic system which employs a liquid developer.
2. Related Art
In an electrophotographic type image forming apparatus, a fixing device with respect to a transfer material is used. In the fixing device as described above, a heating roller and a pressure roller constitute a fixing roller pair, and a nip portion is formed at a position at which the heating roller and the pressure roller come into abutment with each other. The heating roller is provided with an electric heating unit such as a heater lamp. A recording medium (transfer material) such as a recording paper is allowed to pass through the nip portion while being heated and pressurized, so that fixation is achieved.
JP-A-2002-365967 discloses an image forming apparatus having two or more fixing roller pairs disposed along the direction of transport of the recording medium. Referring to FIG. 1 in JP-A-2002-365967, a first fixing unit A is provided on the upstream side in terms of the direction of transport of a transfer material P, and a second fixing unit B on the downstream side. A fixing roller 1 employed as a heating member in the first fixing unit A is provided with a elastic layer 12 formed of silicon rubber on a core metal 11 formed of aluminum, and a mold release layer 13 formed of PFA (tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer) on the elastic layer 12. Also, a fixing roller 3 employed as a heating member in the second fixing unit B is provided with a elastic layer 32 formed of silicon rubber on a core metal 31 formed of aluminum. A mold release layer 33 is formed of PFA on the elastic layer 32.
In this type of image forming apparatus, a configuration employing a liquid developer is known. FIG. 8 is an explanatory drawing showing toner formed on the surface of the transfer material when the transfer material is in an unfixed state in schematic form in an example of the image forming apparatus employing the liquid developer. In FIG. 8, reference numeral 97 designates a transfer material, reference numeral 75 designates a carrier, and reference numeral 76 designates a solid content.
FIGS. 9A to 9D are explanatory drawings showing a state of the toner formed on the surface of the transfer material in the related art as disclosed in JP-A-2002-365967. In the configuration in the related art, the heating rollers provided in a plurality of stages each include a elastic layer provided on a core metal and a mold release layer formed on the elastic layer. FIGS. 9A and 9B show states of the toner in the first fixing unit on the upstream side in terms of the direction of transport of the transfer material. A heating roller 83x is formed of a elastic material. FIG. 9A shows a state in which aggregation of the melted solid content 76 is insufficient. FIG. 9B shows a state in which the melted solid content 76 has started to aggregate, but the carrier 75 and the melted solid content 76 are mixed and hence the solid content 76 cannot be liquidized.
FIG. 9C shows a state of the toner in a second fixing unit. In FIG. 9C, aggregation of the solid content 76 which is further melted from the state shown in FIG. 9B is increased, but liquidization of the solid content 76 is still insufficient. FIG. 9D shows a state of the toner after the fixation in the second fixing unit. In this example, since layer separation between the carrier 75 and the melted solid content 76 is insufficient, the state of a film of the toner formed on the surface of the transfer material is not good, and a shape different from the shape of the surface of the transfer material is assumed.
In this manner, in the configuration of the heating roller described in JP-A-2002-365967, when the liquid developer is used in the image forming apparatus, a film is formed with the carrier remaining in the toner solid content at the time of heating in the fixing device. Therefore, there is a problem that the gloss of a part where the solid content exists is significantly different from the original gloss of the transfer material, and hence a low quality image is formed.